vi] THE BACTERIA OF MILK 97 



forming, and are consequently extremely difficult to 

 destroy, since many of their spores can resist a tem- 

 perature of about 115 C. (240 Fahr.). It is owing 

 to their presence in it that milk is so difficult to 

 completely sterilise. 



There are one or two other forms of fermentation 

 which do not come under any of the classes above 

 mentioned. Among them is alcoholic fermentation. 



Alcoholic Fermentation. This kind of fermenta- 

 tion does not often occur in milk ; but although it is 

 true that milk does not readily undergo alcoholic fer- 

 mentation, yet it can be induced 1 ^; and in certain 

 parts of the world two beverages made from milk 

 which has undergone this kind of fermentation 

 have been long in use. These beverages are kephir 

 and koumiss. 



Kephir. Kephir has long been used in the 

 Caucasus, and is made from cow's milk as a rule. 

 Investigations were first made into its nature as early 

 as the close of last century, and since then it has 

 been made the subject of further research. It is 

 made by the help of a special ferment known as 

 "kephir grains." This ferment is in the form of 

 hard, yellow, granular lumps about the size of a pea. 

 By soaking these kephir grains in water and then 

 adding them to milk, alcoholic fermentation is 

 speedily induced, and the beverage is ready for use 

 in the course of two or three days. The object of 



7 



